Apparatus and methods for testing surface properties of a material

ABSTRACT

An apparatus for testing the surface properties of a material is disclosed. The apparatus includes an indentor which is suspended above the surface of a sample and dropped onto the sample. Several testing methods are also disclosed. Surface toughness is measured by dropping the indentor from a variable height and inspecting the surface for failure. Thin film strength is tested by making several drops from various heights and inspecting the surface for flaking of the layer. In addition, damping capacity can be measured by comparing the kinetic energy of the resulting from the drop of the indentor to the strain energy measured by a probe on the opposing surface of the sample.

This is a divisional of co-pending application Ser. No. 096,185 filed onSept. 11, 1987, now U.S. Pat. No. 4,776,202.

BACKGROUND OF THE INVENTION

Many computers include at least one hard disk for data storage and aread/write head for writing data onto the hard disk or reading data fromthe disk. Small computers, such as personal computers, may have a harddisk and read/write head combination. Larger more powerful computersalmost always are equipped with a number of these hard disks andread/write heads. There are also many data storage devices that use harddisks and read/write heads.

Generally as the computer operates the read/write heads fly over thehard disk. However, at times the read/write head contacts the disk, suchas during takeoff and landing of the read/write head.

An unintended contact between the head and the disk that results in adata loss is called a head crash. Head crashes result from a variety ofreasons and are always undesirable in a data storage device.

Generally, data storage devices can operate for many hours before a headcrash occurs. Head crashes can occur prematurely. The materials whichmake up the read/write head or hard disk are one possible cause for apremature head crash. The majority of the head is made from a ceramicmaterial. The hard disk is a metal disk substrate with several thinlayers of other materials which hold individual magnetic charges andlubricate the head to disk interface. If the ceramic material making upthe head is too hard or brittle it can chip during takeoff or landing.If the bonding between the layers on the metal substrate is poor a layermay flake off when the head lands or takes off. In either case, the chipor flake could possibly lie on the disk and trip the read/write head ona subsequent rotation causing a head crash. One type of hard disk has acarbon overcoating. If the carbon overcoating is too soft it canaccumulate on the head, eventually begin to drag and cause a prematurehead crash.

To date, the methods and apparatus used to test the materials making upthe head and disk have been inadequate. Most test the bulk properties ofa material rather than the surface properties. In addition, no testerseems to simulate the contact that occurs between the head and the diskduring landing or takeoff.

As a result, there is a need for a tester capable of testing the surfaceproperties of a material. Furthermore, there is a need for a tester andmethods for using the tester that simulates the conditions that occurwhen the head and the disk contact during takeoff and landing.

SUMMARY OF THE INVENTION

A tester capable of testing the surface and composite thin filmmicro-properties of a material is disclosed, which is different than themeasurement of bulk properties of materials. The tester is comprised ofa sample holder, an indentor, and a mechanism for holding the indentorthrough a range of desired heights above a sample. The mechanism alsoreleases the indentor so that kinetic energy is imparted upon thesurface of the sample. Several testing methods including determinationof the adhesion strength, the relative damping, capacity, micro-impactstrength, micro-toughness, and surface deformation pattern at thesurface are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference may behad to the accompanying drawings in which:

FIG. 1 is a side view of the inventive tester.

FIG. 2 is a isometric view of a knoop indentor.

FIG. 3 is a view showing a cutaway of the hard disk and showing thearrangement for testing the damping capacity.

FIG. 4 is a side view of a multilayered sample. The arrows in FIG. 4show the direction of forces and energy produced after the indentor isdropped onto the sample.

FIG. 5 is an enlarged view of an indentation in the surface of a sample.

These drawings are not intended as a definition of the invention but areprovided solely for the purpose of illustrating the preferred embodimentof the invention described below.

DETAILED DESCRIPTION OF THE INVENTION

An inventive tester 10 for testing the surface properties of a testspecimen or sample 12 is shown in FIG. 1. The inventive tester 10includes a base 14, an indentor 15 and means for suspending and guidingthe indentor. The base 14 includes a table 16 which holds the sample 12.The table 16 is attached to a positioning means such as pair ofmicrometers 18 and 20. The micrometer 18 controls the position of thetable 16 in a direction parallel with a first edge of the base 14. Themicrometer 20 controls the position of the table 16 in a directionsubstantially perpendicular to the first edge of the base 14.

Attached to the base 14 is a back 22. A support arm 24 extends out fromthe back 22. Attached to the arm 24 is the means for suspending andguiding the indentor which includes a solenoid winding 26, a cylindricalcore 28 and a guide 30. The core 28 includes an opening 29 which passesthrough its length. The opening 29 is also cylindrical in shape and theaxis of the opening 29 and the core 28 are the same. The core 28 ispositioned within the solenoid winding 26. The guide 30 is tubular andmade of a nonmagnetic material such as glass or plastic. The guide 30fits within the solenoid winding 26. The guide 30 is adjustable and canbe extended beyond the end of the winding 26 toward the base 14.

The arm 24 is adjustably attached to the back 22. A micrometer 32 isused to move the arm 24 up and down the back 22 so that the indentor 15can be dropped onto a sample 12 from a range of desired heights. In thepreferred embodiment the drop height ranges from 0.0001" to 0.25". Thedrop heights are relatively small so that the indentor 15 makesindentations mainly in the surface of the sample 12.

The indentor 15 is basically cylindrical in shape and has a suspensionend 34 and an indentor end 36. The suspension end 34 includes acylindrical rod 38 made of a high magnetic permeability material.Centered in and extending out from the top of the rod 38 is a needle 40having an opening 42 therein. A thread 44 is secured through the opening42. The thread 44 also passes through the opening 29 in the core 28. Thethread 44 is of ample length so that a portion extends out the end ofthe core 28 above the support arm 24.

The indentor end 36 can have a variety of shapes to assess the microsurface properties of materials. The knoop-indentor 25 shown in FIGS. 1and 2 is essentially shaped like a pyramid having known dimensions.Generally, variety of indentors having the desired shaped ends forparticular tests will be used rather than one indentor 15 with aninterchangeable head. For example, it is contemplated that severalindentors with different geometrical shaped ends having differentheights will be used on samples having substantially differenthardnesses. Another type of indentor 15 will have a ball-shaped indentorend 36. It should be noted that the indentor 15 may vary in weight.However, to conduct tests to determine the surface properties of asample the indentor will generally weigh between 0.05 grams to 2 grams.The higher range may vary, of course, depending on the material beingtested.

In operation the thread 44 is passed through the opening 29 in the core28. The end of the thread is attached to the needle 40 through theopening 42 therein. The thread 44 is then pulled from the top to raisethe indentor 15.

The indentor is lifted until the needle 40 on the suspension end 34 ofthe indentor 15 extends into the opening 29 of the core 28. At thispoint power is provided to the solenoid winding 26. This, as is wellknown in the art, induces a magnetic field which is concentrated in thecore 28. The rod of high permeability material 38 on the indentor 15 isattracted to the core 28. The indentor 15 is held in place by themagnetic field produced.

The core 28 is made of a material which has high permeability and lowcoercivity. Thus when the power to the winding 26 is turned off, thecore will have very little, if any, residual magnetism. The magneticcouple which suspended the indentor 15 disappears and allows theindentor 15 to fall substantially straight down. The guide 30 isgenerally moved into close proximity to the surface of the sample 12 tokeep the indentor falling substantially straight down through each drop.

Before the indentor 15 is dropped, the sample 12 is placed below theindentor 15 on the table 16. Using the micrometers 18 and 20, the sample12 can be precisely positioned at a desired location. Several testingmethods require the sample to undergo more than one test. Themicrometers 18 and 20 are then used to move the sample 12 so theindentor 15 falls on a new portion of the sample for the subsequenttest.

The force or energy with which the indentor 15 strikes the surface canbe varied by changing the height through which the indentor 15 falls.The height can be easily varied by using the micrometer 32. Themicrometer 32 moves the support area 24 vertically which in turn variesthe height of the suspension means attached thereto. The micrometer 32also allows for slight height adjustments when only slight differencesin the energy imparted on the surface are desired. The micrometer 32also allows one to return to a particular height so as to substantiallyduplicate the energy imparted on the sample 12.

Types of Tests

Several testing methods and their uses are described in the followingparagraphs. It should be noted that these tests are used to compare twomaterials which may come from different vendors or different batches.Consequently, some of the tests do not have specific units.

Damping Capacity Test

FIG. 3 shows the test setup necessary to conduct the damping capacitytest. A damping probe 60 is placed in contact with the test specimen 12at a location 62. The location 62 is on the opposite side of the testspecimen 12 and directly below the point where the indentor 15 contactsthe test specimen 12. The damping probe 60 is a transducer whichmeasures the rate of change in the strain at the location 62.

The kinetic energy imparted on the surface of the test specimen 12causes the material to contract and expand as it travels through thesample 12. It also causes the material at location 62 to expand andcontract. The strain in the material, change in length per unit length,varies over time. The probe 60 measures the rate at which the strain ofthe material at location 62 changes over time. The rate at which thestrain changes is directly related to the energy, known as the strainenergy, in the material. The kinetic energy imparted by the indentor 15can be calculated from the mass of the indentor and the height fromwhich it is dropped. The kinetic energy imparted onto the sample 12 lessthe strain energy detected by the probe 60 yields the energy absorbed bythe specimen 12. The energy absorbed by the specimen 12 determines themicro-damping and energy capacity of the specimen 12. The dampingcapacity test can be used for both the head material and the hard diskmaterial.

Thin Film Adhesion Strength of a Disk

Now turning to FIG. 4, arrows show the direction in which a portion ofthe energy travels after the indentor strikes the surface of the sample12. The arrows indicate that energy travels along the interfaces betweenthe layers of the sample 12. The thin film adhesion strength test is ameasure of the amount of energy the thin films can withstand beforefailing. The specimen 12 undergoes a series of drops by the indentor 15.The height of the indentor 15 is increased and the sample 12 is movedbefore each drop. The result is a series of tests sites or indentationsof which have been subjected to different amounts of kinetic energy. Anadhesive is applied to the surface of the sample 12 adjacent each testsite. A lifting force is then applied to each test site to see if thethin layer of material adjacent the test site lifts off the testspecimen 12. The test site with the maximum drop height of the indentor15 in which the area adjacent the site does not lift off is the site ofinterest. Knowing the height through which the indentor 15 dropped andits weight, the maximum kinetic energy the thin layer of material canwithstand is easily calculated. This test is used to compare theadhesion strength between the layers of a multilayered material. Atypical use may be to compare the adhesion strength between two layersthat were bonded by different processes.

Micro-Impact/Toughness Test for Any Surface

A specimen 12 is mounted to the table 16 of the tester 10. An indentor15 of known geometry is raised to a prescribed height and dropped ontothe surface of the specimen 12. The depth of penetration for theprescribed height indicates the micro-toughness of the materialcomprising the specimen 12.

A knoop type indentor 15, shown in FIGS. 1 and 2, has a known geometryfrom which the depth of penetration can be calculated. The length of theindentation is measured using a microscope. From the known geometry aratio between the measured length and the depth of penetration isdetermined.

This test can be used to compare the micro-toughness of two materials.For example, after a first sample 12 undergoes a drop test, a sample 12from a different vendor or from a different batch can then be mountedonto the tester 10. The indentor 15 is then dropped from the same heightand the length of the indentation can be measured. The depth ofpenetration can then be determined and compared to the previous sample.The sample having the indentation of shorter length has a smaller depthof penetration and is therefore tougher.

Fracture Toughness Metalic Materials

The fracture toughness for a plastic material is the ability of a sample12 to absorb energy and deform plastically. The sample is subjected todrops of the indentor 15 from various heights as the specimen 12 ismoved between drops or tests. Preferably, the height is increased as thespecimen is moved in one direction. This produces a line of test siteswhich have been subjected to an increasing amount of kinetic energy.

The area adjacent the sites are then inspected using a microscope tolook for evidence of deformation, pattern, shape and size. FIG. 5 showsan enlarged view of the surface of a sample which has undergone plasticdeformation. An indentation 46 is shown in the sample 12. Around theindentation 46 are ridges 48 which indicate plastic deformation of thesurface.

The site having evidence of plastic deformation therewith indicates theamount of kinetic energy that can be absorbed at the surface of thesample 12 before deformation takes place as a measure of fracturetoughness. This test is also used to indicate which of several sampleshas the highest tolerance toward kinetic energy or impact energyimparted at the surface.

Fracture Toughness Ceramics & Superconducting Materials

The same test procedure is followed when testing such material specimen12 such as the ceramic head material. Once a series of test sites isproduced with variable amounts of kinetic energy associated therewith,the specimen 12 is inspected using a microscope. The difference from theprevious test is that the area adjacent the test sites are inspected forsigns of chipping, and cracking rather than plastic deformation.

What we claim is:
 1. A method for determining the impact energy capableof being withstood by a brittle sample surface comprising the stepsof:dropping an indentor having a known mass onto the sample from apredetermined height above the sample; inspecting the sample for failureat the surface in the area adjacent the indention by determining thepresence of chips in the area adjacent the indentation; and determiningthe energy imparted on the surface from the known mass and thepredetermined height through which the indentor was dropped.
 2. Themethod of claim 1 further including the steps of:producing a series ofindentations having differing amounts of energy associated with eachindentation which includes the steps of: raising the indentor to adifferent height from the previous height through which the indentordropped; repositioning the sample; dropping the indentor onto the sampleto produce an indentation on the surface of the sample; and determiningthe impact energy capable of being withstood by the surface of thesample by noting the lowest indentor drop height which produced anindentation having failure on the surface adjacent the indentation.
 3. Amethod for determining the impact energy capable of being withstood by aductile sample surface comprising the steps of:dropping an indentorhaving a known mass onto the sample from a predetermined height abovethe sample; inspecting the sample for failure at the surface in the areaadjacent the indention by determining the presence of plasticdeformation in the area adjacent the indentation; and determining theenergy imparted on the surface from the known mass and the predeterminedheight through which the indentor dropped.
 4. The method of claim 3further including the steps of:producing a series of indentations havingdiffering amounts of energy associated with each indentation whichincludes the steps of: raising the indentor to a different height fromthe previous height through which the indentor dropped; repositioningthe sample; dropping the indentor onto the sample to produce anindentation on the surface of the sample; and determining the impactenergy capable of being withstood by the surface of the sample by notingthe lowest indentor drop height which produced an identation havingfailure on the surface adjacent the indentation.